@Sicario2 - Tier 1 I know quite a few people with medically diagnosed people. I've been rung by my mate a few hours after he tried to kill himself. He's a partner at a huge firm and is very "successful".
He went on to establish this - https://citymha.org.uk/ - it's worth a look.
I know people who were abused as children who are only now dealing with the mental ramifications because how does a 5 year old deal with sexual abuse? Those encounters have led to depression, anxiety, OCD. There is plenty of medical research to support this. Those people were surrounded by the most loving and caring and "successful" people.
Depression can hit anyone. It's a disease, not a weakness. There is plenty of evidence to support this statement if you are interested enough to go and have a look. You don't need to have had trauma to suffer depression. it can affect anyone, for unknown reasons.
And yes, part of treating depression involves forcing yourself to get on with your life. Exercise helps. So does medication. And time. Like any other disease.
But it's not something that is treated by just telling yourself to get on with things.
It's really important that people like Dele Alli speak up and tell their stories. If it helps one other person then that's wonderful.
Good on Dele Alli and good luck to him, and to anyone else suffering out there.